1. © 2011 Pearson Education, Inc. Earth: An Introduction to Physical Geology, 10e Tarbuck & Lutgens

2. © 2011 Pearson Education, Inc. Magma, Igneous Rocks, and Intrusive Activity Earth, 10e - Chapter 4 Stan Hatfield Southwestern Illinois College

3. © 2011 Pearson Education, Inc. General Characteristics of Magma Igneous rocks form as molten rock cools and solidifies. General characteristics of magma: Parent material of igneous rocks Forms from partial melting of rocks Magma at surface is called lava.

4. © 2011 Pearson Education, Inc. General Characteristics of Magma General characteristics of magma: Rocks formed from lava are extrusive, or volcanic rocks. Rocks formed from magma at depth are intrusive, or plutonic rocks.

5. © 2011 Pearson Education, Inc. General Characteristics of Magma The nature of magma Consists of three components: 1.Liquid portion = melt 2.Solids, if any, are silicate minerals. 3.Volatiles are dissolved gases in the melt, including water vapor (H 2 O), carbon dioxide (CO 2 ), and sulfur dioxide (SO 2 ).

6. © 2011 Pearson Education, Inc. General Characteristics of Magma Crystallization of magma Cooling of magma results in the systematic arrangement of ions into orderly patterns. Silicate minerals result from crystallization in a predictable order. Texture is the size and arrangement of mineral grains.

7. © 2011 Pearson Education, Inc. Igneous Textures Texture is the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals. Factors affecting crystal size: Rate of cooling – Slow rate = fewer but larger crystals – Fast rate = many small crystals – Very fast rate forms glass.

8. © 2011 Pearson Education, Inc. Igneous Textures Factors affecting crystal size: Percentage of silica (SiO 2 ) present Dissolved gases

9. © 2011 Pearson Education, Inc. Igneous Textures Types of igneous textures Aphanitic (fine-grained) texture – Rapid rate of cooling – Microscopic crystals – May contain vesicles (holes from gas bubbles) Phaneritic (coarse-grained) texture – Slow cooling – Large, visible crystals

10. © 2011 Pearson Education, Inc. Phaneritic Texture

11. © 2011 Pearson Education, Inc. Igneous Textures Types of igneous textures Porphyritic texture – Minerals form at different temperatures. – Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass). Glassy texture – Very rapid cooling of lava – Rock is called obsidian.

12. © 2011 Pearson Education, Inc. Igneous Textures Types of igneous textures Pyroclastic texture – Fragmental appearance produced by violent volcanic eruptions – Often appear more similar to sedimentary rocks Pegmatitic texture – Exceptionally coarse-grained – Form in late stages of crystallization of granitic magmas

13. © 2011 Pearson Education, Inc. Pegmatitic Texture

14. © 2011 Pearson Education, Inc. Glassy Texture—Obsidian

15. © 2011 Pearson Education, Inc. Igneous Compositions Igneous rocks are composed primarily of silicate minerals. Dark (or ferromagnesian) silicates – Olivine, pyroxene, amphibole, and biotite mica Light (or nonferromagnesian) silicates – Quartz, muscovite mica, and feldspars

16. © 2011 Pearson Education, Inc. Igneous Compositions Granitic versus basaltic compositions Granitic composition – Light-colored silicates – Termed felsic (feldspar and silica) in composition – High silica (SiO 2 ) content – Major constituent of continental crust

17. © 2011 Pearson Education, Inc. Igneous Compositions Granitic versus basaltic compositions Basaltic composition – Dark silicates and calcium-rich feldspar – Termed mafic (magnesium and ferrum, for iron) in composition – Higher density than granitic rocks – Comprise the ocean floor and many volcanic islands

18. © 2011 Pearson Education, Inc. Igneous Compositions Other compositional groups Intermediate (or andesitic) composition – Contain 25% or more dark silicate minerals – Associated with explosive volcanic activity Ultramafic composition – Rare composition that is high in magnesium and iron – Composed entirely of ferromagnesian silicates

19. © 2011 Pearson Education, Inc. Igneous Compositions Silica content as an indicator of composition Crustal rocks exhibit a considerable range— 45% to 70% Silica content influences magma behavior. Granitic magmas have high silica content and are viscous. Basaltic magmas have much lower silica content and more fluid-like behavior.

20. © 2011 Pearson Education, Inc. Igneous Compositions Naming igneous rocks—granitic rocks Granite – Phaneritic – Over 25% quartz, about 65% or more feldspar – Very abundant—often associated with mountain building – The term granite includes a wide range of mineral compositions.

21. © 2011 Pearson Education, Inc. Igneous Compositions Naming igneous rocks—granitic rocks Rhyolite – Extrusive equivalent of granite – May contain glass fragments and vesicles – Aphanitic texture – Less common and less voluminous than granite

22. © 2011 Pearson Education, Inc. Igneous Compositions Naming igneous rocks—granitic rocks Obsidian – Dark colored – Glassy texture Pumice – Volcanic – Glassy texture – Frothy appearance with numerous voids

23. © 2011 Pearson Education, Inc. Igneous Compositions Naming igneous rocks—intermediate rocks Andesite – Volcanic origin – Aphanitic texture Diorite – Plutonic equivalent of andesite – Coarse-grained

24. © 2011 Pearson Education, Inc. Igneous Compositions Naming igneous rocks—basaltic rocks Basalt – Volcanic origin – Aphanitic texture – Composed mainly of pyroxene and calcium-rich plagioclase feldspar – Most common extrusive igneous rock

25. © 2011 Pearson Education, Inc. Igneous Compositions Naming igneous rocks—mafic rocks Gabbro – Intrusive equivalent of basalt – Phaneritic texture consisting of pyroxene and calcium-rich plagioclase – Significant percentage of the oceanic crust

26. © 2011 Pearson Education, Inc. Classification of Igneous Rocks

27. © 2011 Pearson Education, Inc. Common Igneous Rocks

28. © 2011 Pearson Education, Inc. Igneous Compositions Naming igneous rocks—pyroclastic rocks Composed of fragments ejected during a volcanic eruption Varieties – Tuff = ash-sized fragments – Volcanic breccia = particles larger than ash

29. © 2011 Pearson Education, Inc. Origin of Magma Generating magma from solid rock Role of heat – Temperature increases in the upper crust (geothermal gradient) average between 20 o C to 30 o C per kilometer. – Rocks in the lower crust and upper mantle are near their melting points. – Additional heat may induce melting.

30. © 2011 Pearson Education, Inc. A Typical Geothermal Gradient

31. © 2011 Pearson Education, Inc. Origin of Magma Role of pressure – Increases in confining pressure increases a rock’s melting temperature. – When confining pressures drop, decompression melting occurs. Role of volatiles – Volatiles (primarily water) cause melting at lower temperatures. – Important factor where oceanic lithosphere descends into the mantle

32. © 2011 Pearson Education, Inc. Decompression Melting

33. © 2011 Pearson Education, Inc. Evolution of Magmas A single volcano may extrude lavas exhibiting very different compositions. Bowen’s reaction series Minerals crystallize in a systematic fashion based on their melting points. During crystallization, the composition of the liquid portion of the magma continually changes.

34. © 2011 Pearson Education, Inc. Bowen’s Reaction Series

35. © 2011 Pearson Education, Inc. Evolution of Magmas Processes responsible for changing a magma’s composition Magmatic differentiation – Separation of a melt from earlier formed crystals Assimilation – Changing a magma’s composition by incorporating surrounding rock bodies into a magma

36. © 2011 Pearson Education, Inc. Evolution of Magmas Processes responsible for changing a magma’s composition Magma mixing – Two chemically distinct magmas may produce a composition quite different from either original magma.

37. © 2011 Pearson Education, Inc. Assimilation, Magma Mixing, and Magmatic Differentiation

38. © 2011 Pearson Education, Inc. Evolution of Magmas Partial melting and magma formation Incomplete melting of rocks is known as partial melting. Formation of basaltic magmas – Most originate from partial melting of mantle rocks at oceanic ridges – Large outpourings of basaltic magma are common at Earth’s surface.

39. © 2011 Pearson Education, Inc. Evolution of Magmas Partial melting and magma formation Formation of andesitic magmas – Produced by interaction of basaltic magmas and more silica-rich rocks in the crust – May also evolve by magmatic differentiation

40. © 2011 Pearson Education, Inc. Evolution of Magmas Partial melting and magma formation Formation of granitic magmas – Most likely form as the end product of crystallization of andesitic magma – Granitic magmas are more viscous than other magmas—tend to lose their mobility before reaching the surface. – Produce large plutonic structures

41. © 2011 Pearson Education, Inc. Intrusive Igneous Activity Most magma is emplaced at depth in the Earth. An underground igneous body, once cooled and solidified, is called a pluton. Classification of plutons Shape – Tabular (sheet-like) – Massive

42. © 2011 Pearson Education, Inc. Intrusive Igneous Activity Classification of plutons Orientation with respect to the host (surrounding) rock – Discordant—cuts across sedimentary rock units – Concordant—parallel to sedimentary rock units

43. © 2011 Pearson Education, Inc. Intrusive Igneous Activity Types of intrusive igneous features Dike—a tabular, discordant pluton Sill—a tabular, concordant pluton (e.g., Palisades Sill in New York) Laccolith – Similar to a sill – Lens or mushroom-shaped mass – Arches overlying strata upward

44. © 2011 Pearson Education, Inc. Some Intrusive Igneous Structures

45. © 2011 Pearson Education, Inc. A Sill in the Salt River Canyon, Arizona

46. © 2011 Pearson Education, Inc. Intrusive Igneous Activity Intrusive igneous features Batholith – Largest intrusive body – Surface exposure of 100+ square kilometers (smaller bodies are termed stocks) – Frequently form the cores of mountains

47. © 2011 Pearson Education, Inc. Intrusive Igneous Activity Emplacement of magma Magma at depth is much less dense than the surrounding rock. – Increased temperature and pressure causes solid rock to deform plastically. – The more buoyant magma pushes aside the host rock and forcibly rises in the Earth as it deforms the “plastic” host rock.

48. © 2011 Pearson Education, Inc. Intrusive Igneous Activity Emplacement of magma At more shallow depths, the host rock is cooler and exhibits brittle deformation. – Movement of magma here is accomplished by fractures in the host rock and stoping. Melting and assimilation of the host rock is greatly limited by the availability of thermal energy.

49. © 2011 Pearson Education, Inc. End of Chapter 4